MapBase<K, V> class

This class has a basic implementation of all but five of the members of
Map.
A basic Map class can be implemented by extending this class and
implementing keys, operator[], operator[]=, remove and clear.
The remaining operations are implemented in terms of these five.

The keys iterable should have efficient Iterable.length and
Iterable.contains operations, and it should catch concurrent modifications
of the keys while iterating.

A more efficient implementation is usually possible by overriding
some of the other members as well.